When designing Hybrid vehicles, engineers are facing important challenges due to increased interactions between the main vehicle subsystems and the diversification of both powertrain technologies and architectures. As an example, the range of hybrid transmissions includes serial hybrids, parallel and power split hybrids with variable hybridization rates from micro hybrids (start & go systems) to full hybrids.Because of the increasing number of interactions (mechanical, thermal, electrical, hydraulic, controls) between the vehicle subsystems, design analyses at component level proved to be insufficient to reach best performances and driving pleasure while at the same time decreasing fuel consumption and pollutant emissions. Today, automotive OEM design departments define targets for subsystems in early stage by taking into account individual vehicle attributes. However, several attributes have an impact on different subsystems, indicating the need for a multi-attribute target setting using an appropriate vehicle synthesis approach.In this context, this paper highlights the benefits of Model Based System Engineering using a multi-level and multi-domain system simulation approach for transient and vehicle synthesis analyses.Several numerical simulators at system level has been developed, allowing not only to simulate the impact of new powertrain and vehicle technologies on fuel consumption but also on other attributes like driveability and thermal comfort. These simulators are now mandatory to keep the development time and cost of vehicles into acceptable limits.